Knight — 120 — Dictionary 



Polytypic. — Having several forms, or more than one form. 



Position-effect. — In certain organisms it has been shown that 

 the visible effect of a gene is to some extent dependent on its 

 interactions with its near neighbours in the chromosome and 

 that changes in position due to translocations, inversions, etc. 

 can alter the nature of the outward expression of the gene. 



Postheterokinesis. — A form of meiotic division characterized 

 by failure of the sex chromosome to divide so that it passes 

 to only one pole in the second spermatocyte division, cf. Pre- 

 heterokinesis. 



Post-reduction. — A halving of the chromosome number oc- 

 curring in the second meiotic division as opposed to normal 

 reduction ('pre-reduction') which occurs during the first 

 meiotic division. 



Potence. — The effect of the integrated dominance and inter- 

 action relations of all the polygenic allelomorphs within the 

 possible combinations (Wigan). 



Potency (of a gene). — The capacity of a gene for mani- 

 festing its presence, cf. Expressivity and Penetrance. Partial 

 potency is synonomous with incomplete dominance. 



Pre-adaptation. — The existence of a character which ren- 

 ders an organism potentially able to make use of a changed 

 environment or to extend its original environmental limits. 



Pre-adaptation, Constitutional. — Adaptation in which a 

 species is, by its existing peculiarities, already modified to suit 

 certain modes of life or particular types of environment. 



Pre-adaptation, Mutational. — Adaptation in which a mutant 

 form or natural variety is, ab initio, adapted to certain spe- 

 cial environmental conditions in which it either originates, or 

 into which it might be thrown by chance. 



Precession. — The passing of the sex chromosomes to the 

 poles in meiosis before the autosomes. 



Precocity. — The property of the nucleus beginning prophase 

 before the chromosomes have divided ; characteristic of meiosis. 



Differential . The property of some chromosomes or 



their parts condensing, dividing or pairing in advance of the 

 rest of the complement during prophase (Darlington). 



Precocity Theory. — Darlington's theory that the main 

 difference between meiosis and mitosis depends on the fact 

 that in meiosis the chromosomes condense precociously out of 

 the resting nucleus and therefore they pair (Altenburg, 1945). 



Predetermination. — Non-genetic variations in form and size 

 of seed. 



Preformation. — See under Epigenesis. 



